Method for determining the magnitude of earth's gravity

Abstract

An improved method that directly and continuously determines the magnitudef the earth's gravity in relation to a user platform at one or more selected points on or above the earth's surface. An improved Global Positioning System (GPS) is used to carry out the method and is made up of a plurality of twenty-four (24) continuously orbiting GPS satellites that are arranged into three groups of eight satellites with each group or constellation being disposed in its respective plane of a series of three longitudinal planes. Each plane is arranged in predetermined angular and spatial relation to the earth's equatorial plane and the other planes of the series. Any GPS satellite continuously transmits a pair of encoded RF signals at predetermined L-band frequencies. By virtue of this global arrangement of the 24 satellites, the antenna field of view of a user platform is capable of receiving at any time the encoded signals of at least six satellites. A geodetic receiver on the platform together with a receiver microprocessor, a navigation microprocessor, and a microcomputer progressively and continuously process signals received from the satellites for the ultimate purpose of comparing GPS dtermined vertical acceleration (that is normal to the earth's modeled ellipsoid) with its platform-measured gravimeter acceleration (that is normal to the earth's geoid) so as to determine, with greater accuracy, the gravity at one or more selected points. By reason of the improved GPS systems carrying out the method of continuously and directly determining gravity with greater accuracy at one or more points, a given area can be readily surveyed in relation to the user platform as well as enabling corrections to inertial navigation equipment and enhancing the guidance of weaponry (missiles, etc.) during firing or launch.

What is claimed is: 
     
       1. A method for directly and continuously determining the magnitude of the gravity at one or more selected points in relation to the earth's surface, the method comprising the steps of: continuously transmitting encoded RF signals from a plurality of Global Positioning System (GPS) earth-orbiting satellites, such that the traasmitted RF signals of each GPS earth-orbiting satellite are a pair of encoded RF signals at predetermined frequencies that are directed in a radial direction towards the earth's surface, all GPS satellites of the plurality of satellites progressively and continuously advancing about the earth in relation to the field of view of the antenna of a user platform of the GPS system;   continuously and selectively processing the encoded RF signals received by the platform at one or more selected points from a series of GPS satellites of the plurality in the field of view of the antenna so as to initially determine the Doppler pseudo range between each satellite of the series and the platform and the radial relative velocity therebetween,   further processing the encoded RF signals of the series of satellites so as to select and track an optimum group of satellites of the series that will provide minimal error in determining the navigation position of the platform and the relative radial velocity of the platform with respect to each satellite of the optimum group,   also further processing the encoded RF signals of the optimum group of selected and tracked satellites so as to determine the relative radial acceleration of the platform with respect to each satellite thereof and to resolve the radial acceleration of the platform with respect to each satellite into the GPS vertical acceleration thereof,   measuring the vertical acceleration of said platform at one or more selected points by way of a gravimeter accelerometer arrangement mounted on the platform, and   also still further processing the encoded RF signals of the optimum group so as to compare the vertical acceleration of the platform with the GPS vertical acceleration of the platform, the comparison providing a difference that indicates the magnitude of the gravity in relation to the platform.   
     
     
       2. A method as in claim 1 in which each satellite of said GPS transmits the pair of encoded RF signals at predetermined L-band frequencies of 1227.6 megaHertz and 1575.42 megaHertz. 
     
     
       3. A method as in claim 1 in which the platform is mounted on a marine vessel. 
     
     
       4. A method as in claim 1 in which the platform is mounted on an aircraft. 
     
     
       5. A method as set forth in claim 1 in which the platform is mounted on a support that is movable along the earth's surface. 
     
     
       6. A method as in claim 1 wherein the plurality of GPS satellites are comprised of twenty-four satellites with the plurality of satellites being arranged in three separate groups of eight, each group being disposed in its respective longitudinal plane in relation to the earth, each longitudinal plane being one of a series of three longitudinal planes and also being arranged in spaced angular relation to the other longitudinal planes of the series and the equatorial plane of the earth; and with the satellites of each group of eight moving in a common approximately circular orbital path in their respective longitudinal plane. 
     
     
       7. A method as set forth in claim 6 wherein immediately adjacent longitudinal planes of the series of longitudinal planes define an angle of approximately 120° therebetween. 
     
     
       8. A method as set forth in claim 6 wherein each longitudinal plane of the series and the earth equatorial plane define an angle of approximately 63° therebetween. 
     
     
       9. A method as set forth in claim 6 wherein each satellite of the GPS is disposed at an altitude of approximately 20,000 kilometers above the earth's surface. 
     
     
       10. A method as set forth in claim 6 wherein the group of satellites in each longitudinal plane of the series are equally spaced from each other along their orbital path. 
     
     
       11. A method as set forth in claim 6 wherein each satellite orbits about the earth in approximately twelve hours. 
     
     
       12. A method as set forth in claim 1 wherein the field of view of the antenna is of conical configuration in an upward and outward divergent direction toward the horizon such that the solid angle of the conical field of view adjacent the antenna is approximately 160°. 
     
     
       13. A method as set forth in claim 1 wherein the steps of processing the encoded RF signals is effected by a geodetic receiver connected to the antenna, a receiver microprocessor, a navigation microprocessor, and a gravity microcomputer; and wherein the microcomputer and the navigation microprocessor are operatively associated with the receiver microprocessor. 
     
     
       14. A method as set forth in claim 1 wherein the magnitude of the gravity has an error of less than 5 milligals when a user platform has a speed of less than 100 meters/second. 
     
     
       15. A method as set forth in claim 1 wherein the step of measuring the vertical acceleration of said platform is effected by the gravimeter accelerometer arrangement being made up of a plurality of three linear accelerometers.